Technetium compounds

Technetium compounds are the most important radio-pharmaceuticals in use today. Over 80% of all... 

Technetium compounds with amine/thioether coordination are the cationic trans-dioxotechnetium(V) complexes [Tc02(N2S2)]+. The complex in which N2S2 is l,4-dithia-8,ll-diazacyclotetradecane was prepared via an exchange reaction of NBu4[TcOBr4] with the ligand and fully characterized by X-ray crystal structure determination [109,110]. The coordination around technetium... 

Rhenium chemistry will not be discussed explicitly in this contribution. Various excellent reviews on the progress of organometallic rhenium chemistry have been published over recent years. [7] Rhenium is only mentioned when its comparison to new technetium compounds is interesting in the present context and the focus is on significant differences in the behaviour of the two homologs. 

Most of the substitution reactions with the homoleptic Tc(I) isocyanide complexes presented in the preceding section had to be performed at elevated temperatures and were often characterized by low yield. The reason for this behaviour is the exceptionally high kinetic and thermodynamic stability of this class of compounds. From this point of view, 4a are not very convenient or flexible starting materials, although they are prepared directly from 3a in quantitative yield. The exceptionally high kinetic and thermodynamic stability is mirrored by the fact that it was not possible to substitute more than two isocyanides under any conditions. On the other hand, oxidation to seven-coordinated Tc(III) complexes occurs very readily. Technetium compounds of this type, which are not expected to be very inert, could open up a wide variety of new compounds, but this particular field has not been investigated very thoroughly. A more convenient pathway to mixed isocyanide complexes that starts with carbonyl complexes of technetium will be described in Sects. 2.3 and 3.2. 

Metal-metal (M-M) bonds, first noted in the early sixties, occur in several thousand transition-metal compounds [1]. Complex technetium compounds and compounds with M-M bonds (clusters) have been studied more extensively than many other classes of inorganic compounds. Increasing interest in technetium compounds is due to the practical uses of the "mTc isotope, which ranks first among radioactive isotopes used in nuclear medicine diagnostics [2-4]. On the other hand, technetium clusters are an interesting object for theoretical studies, because until recently, they were the only compounds in which the presence of these anomalous chemical bonds was thought possible. 

All the current methods of synthesizing of technetium compounds (as well as most compounds of other metals [1,11]) with M-M bonds involve reductions of complexes of the central atom in the higher oxidation states with further (or... 

The first two groups are distinguished by the fact that autoclave syntheses are used mainly by the Soviet (or Russian) cluster school [1,2,11,16-22], while the American and other schools do not use autoclave techniques [1,23-35]. This distinction is historical and applies not only to technetium compounds, but also to other cluster compounds of d-transition elements [36-41]. Cotton et al. [28] did use an autoclave method once, but it was not further developed [1]. 

The majority of cluster technetium compounds are subject to thermal decomposition topochemically (i.e. their decomposition reaction occurs in the solid phase), [H(H20)2]2 [Tc8Br4( -Br)8]Br2 being an exception. This compound melts before decomposition (at 610-620 °C), which is good evidence in favour of the molecular crystalline structure of its dehydrated form [Tc8Br4(/i-Br)8]Br2. ... 

Thus, the investigations performed have shown that cluster fragments in technetium compounds are thermally rather stable. However, in a number of compounds thermal decomposition occurs at comparatively low temperatures, which is mainly related to the instability of organic cations and the weak bonding of technetium with organic ligands. 

Table 4 presents the calculated results of the effective charges on technetium atoms in technetium compounds, arrived at by using various theoretical approximations. In technetium compounds with M-M bonds and formal technetium oxidation states 2.0 + and lower, Zeff is less than 1.0 +, whatever the... 

Fig. 10. Schemes of MO s for technetium compounds according to the results of extended Huckel calculations [58]...

Omori, T. Substitution Reactions of Technetium Compounds. 176, 253-274 (1996). Ostrowicky, A., Koepp, E., Vogtle, F. The Vesium Effect Synthesis of Medio- and Macrocyclic Compounds. 161, 37-68 (1991). 

The periodic trends that distinguish the chemical behavior of rhenium and technetium are manifested in differences between the biological behavior between analogous rhenium and technetium compounds. The comparison can shed light on the mechanism of accumulation in target... 

Technetium compounds of the oxidation states 0 to -1-7 have hitherto been isolated, pertechnetates being the most stable. Also, compounds of quadrivalent technetium are fairly stable. Oxidation states lower than -1-4 can easily be oxidized to the quadrivalent or heptavalent state, whereas penta- and hexavalent technetium readily disproportionate into the hepta- and quadrivalent states. 

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